1. Field of the Invention
The present disclosure relates to a white organic light emitting device which has excellent color purity and improved light emissive efficiency, and which can be manufactured using a simple manufacturing process.
2. Description of the Related Art
Organic light emitting devices are self-emissive devices that function through the use of electroluminescence thereby having a high degree of pattern recognition ability. In addition, organic light emitting devices are complete, solid, devices thereby having excellent impact resistance. As a result, organic light emitting devices are the focus of considerable attention for their potential application to all kinds of display devices.
Organic light emitting devices have a basic structure comprising an anode, an organic emissive layer, and a cathode, and can further comprise a hole injection layer, a hole transport layer, an electron injection layer, or the like. Examples of different structures include, an anode/hole injection layer/hole transport layer/organic emissive layer/cathode structure, and an anode/hole injection layer/hole transport layer/organic emissive layer/electron injection layer/cathode structure.
The development of organic light emitting display devices, in particular, white organic light emitting devices, has been an ongoing focus of research activity.
White organic light emitting devices are organic light emitting devices that emit white light, and can be used for various applications such as paper-thin light sources, backlights for liquid crystal display devices, or light sources for full-color display devices employing color filters.
Methods of forming an emissive layer for a white organic light emitting device can be categorized into two types. One is a method of forming a single emissive layer, and the other is a method of forming multiple emissive layers.
A single emissive layer can be prepared using a single material or, by doping or blending at least two types of materials. For example, the single emissive layer can be formed using red and green dopants with a blue host, or by using red, green and blue dopants with a host material having a large band gap energy. However, in these examples, energy transfer into the dopant is incomplete. Alternatively, the single emissive layer can be formed using a bipolar host material having a red, green, or blue luminescence moiety. However, in this case, the white balance cannot be easily adjusted.
A white organic light emitting device comprising multiple emissive layers can be categorized into both a 3-wavelength, white organic light emitting device, comprising a red emissive layer, a green emissive layer and a blue emissive layer; and a 2-wavelength white organic light emitting device, using colors which are complementary to either red, green or blue.
In the case of the 2-wavelength, white organic light emitting device using complementary colors for either red, green or blue, a high degree of energy transfer efficiency can be obtained. However, one of the drawbacks to the 2-wavelength method is that white is obtained through the use of complementary colors, and consequently it is very difficult to achieve a full range of colors using color filters, and thus the range of colors that can be expressed is narrow. Meanwhile, in the case of the 3-wavelength white organic light emitting device, a uniform spectrum of the three colors, that is, red, green, and blue, cannot be achieved due to the energy transfer between molecules, and consequently the light emission efficiency is still low.
Korean Patent Publication No. 2005-0028564 discloses a method of manufacturing a white organic light emitting device comprising: doping any one pigment selected from green and red, on a portion or whole portion, of any one of a hole transport layer and an electron transport layer, that are formed on upper and lower surfaces of a blue emissive layer; and doping the other one of the green and red pigment on the other layer of the hole transport layer and the electron transport layer. In addition, Japanese Patent Laid-Open Publication No. 2005-150084 discloses a white organic light emitting device in which a double hole blocking layer, comprising a first hole blocking layer, a hole transport layer, and a second hole blocking layer, is formed between an anode and an emissive layer, thereby having high color purity and luminescence in spite of the structure of the emissive layer which comprises a green emissive layer, a blue emissive layer and a red emissive layer which are formed in that order. The white organic light emitting device is manufactured using a simple manufacturing process, but it still has low level of light emission efficiency and color purity.
U.S. Patent Publication No. 2003/0189401, filed by IMES (International Manufacturing and Engineering Services., Co., Ltd.), discloses an organic electroluminescent device, capable of emitting white light, comprising at least two light emitting units formed between a cathode and an anode, that each comprise at least one emissive layer, wherein the light emitting units are separated by at least one charge generating layer. The white organic light emitting device has a tandem structure such that a charge generating layer (CGL) is formed between the light emitting units, as illustrated in FIG. 1A. The white organic light emitting device has an improved light emission efficiency, but the color adjustment is problematic due to an overall light interference effect produced as a result of an increase in the thickness of the emissive layers.
U.S. Patent Publication No. 2006/0040132 discloses a white organic light emitting device having a tandem structure such that a plurality of light emitting units are formed between a cathode and an anode, a connector is formed between the adjacent light emitting units, and each of the light emitting units emit white light. The white organic light emitting device (illustrated in FIG. 1B), has excellent light emission efficiency. However, the manufacturing process for this device is complex, and due to the long light path of the device the, color adjustment is also difficult.